Chemical vapor deposition (CVD) is a well known method for depositing and forming a protective coating on substrates. Typically, the substrates are loaded into a reaction furnace (reactor), heated to a suitable reaction temperature, and exposed in the reactor to one or more elevated temperature gaseous reactant streams that react with the substrate surfaces to deposit a coating or layer thereon. The CVD deposited coating or layer can be reacted with the substrate by suitable heating in the reactor to form a protective diffusion coating thereon; e.g., a high temperature oxidation and corrosion resistant nickel or cobalt aluminide coating on nickel or cobalt base superalloy substrates as described in the Gauje U.S. Pat. No. 3,486,927.
As illustrated in that patent, a gaseous reactant stream may be formed in-situ inside the reactor for reaction with the substrates. Alternately, the gaseous reactant stream may be formed outside the reactor in a heated reactant generator and continuously introduced into the reactor in a carrier gas, such as a reducing or inert gas, so as to pass over the substrates. After passing over the substrates, the carrier gas and any excess, unreacted gaseous reactant are exhausted from the reactor to maintain a continuous gas flow therethrough over the substrates.
In one particular CVD coating apparatus, a plurality of substrates to be coated are fixtured about a gas distribution conduit network within the reactor, and the gaseous reactant stream, such as a metal chloride or fluoride gas, in a reducing or inert carrier gas, is formed external of the reactor in a reactant generator and is metered into the reactor via the conduit network for contact with exterior surfaces of the substrates. A separate gaseous reactant generator external of the reactor and separate distribution conduit network can be optionally provided to supply and meter another gaseous reactant stream into the reactor for contacting interior surfaces of the substrates if they are hollow. Both external and internal coatings can thereby be concurrently formed on hollow substrates, such as hollow gas turbine engine blades, using the CVD apparatus.
In this particular apparatus, the gaseous reactant streams (external and optional internal coating streams) are distributed by respective elongated distribution conduits extending through the heated reactor chamber to different coating zones disposed at axial intervals along each distribution conduit. The gaseous reactant streams are thus heated as they are distributed in the heated reactor chamber to the different coating zones such that the streams may exhibit substantially different temperatures and thus different chemical reactivities toward the substrates to be coated at different zones. Such different coating gas reactivities can result in formation of CVD coatings which are non-uniform in composition and thickness from one coating zone to the next.
It is an object of the present invention to provide an improved CVD apparatus and method wherein the reactivity of the gaseous reactant stream (i.e., activity of a particular chemical species in the reactant stream) is controlled at different coating zones in a heated reactor chamber in a manner to accommodate substantially different temperatures present at the coating zones to produce CVD coatings exhibiting improved uniformity in thickness and composition from one coating zone to the next.
It is another object of the present invention to provide an improved CVD apparatus and method wherein the reactivity of the gaseous reactant stream is controlled at different coating zones in a heated reactor chamber by providing at different coating zones having substantially different reactant stream temperatures, a reactivity-altering material whose composition is selected in dependence on the temperature of the reactant stream at the zones to alter the reactivity of the reactant stream upon contact therewith in a manner to provide substantially the same reactant reactivity at all coating zones.